For example, a three-month note maturing at $100 is issued at $98. Between issuance and maturity, the value of the bond will increase until it reaches its full value of $100, which is the amount that will be paid at maturity.
Accretion can be accounted for in a straight-line method, whereby the increase is evenly spread throughout the term, or by constant interest, whereby the increase is heaviest closest to maturity.